In the wet smelting of nickel from a nickel oxide ore, as a sulfidation step, a treatment is carried out in which a sulfidation reaction is caused under pressure by using a leachate containing nickel and cobalt as a sulfidation starting solution so that a nickel and cobalt mixed sulfide is manufactured while impurities are separated. By causing the sulfidation reaction under pressure in this way, particularly, zinc, copper, and the like can be separated and such a treatment is practically used.
Herein, in the sulfidation step, mainly, a reaction represented by the following Reaction Formula (i) proceeds. As represented in Reaction Formula (i), in the sulfidation step, the [H+] ion concentration in the solution increases according to the progress of the reaction, and the pH decreases according to the progress of the reaction.H2S+NiSO4→H2SO4+NiS  (i)
However, when the pH of the solution decreases according to the progress of Reaction Formula (i), generated NiS is redissolved, and thus the nickel concentration in the solution at a terminating point of the reaction increases and the nickel recovery rate is deteriorated. Therefore, in a case where the pH of the sulfidation starting solution is low, the pH of the end solution also becomes lower, and thus the nickel recovery rate decreases.
Further, in the sulfidation step, when the amount of iron ions, which are more difficult to separate than other metals, is increased, a reaction represented by the following Reaction Formula (ii) is promoted, and similarly to the sulfidation of nickel, the [H+] ion concentration in the solution increases and the pH decreases according to the progress of the reaction.H2S+FeSO4→H2SO4+FeS  (ii)
Regarding the relationship between the iron ions concentration in the sulfidation starting solution and the nickel ions in the end solution, the amount of nickel ions in the end solution increases according to the increase of the iron ions concentration. That is, this means that the nickel recovery rate decreases according to the increase of the iron ions concentration. Since the iron ions concentration in the sulfidation starting solution depends on an operation parameter in a step prior to the sulfidation step in the nickel oxide ore hydrometallurgical method, the iron ions concentration in the starting solution at the time of starting the sulfidation step is not uniform but is in a “nonuniformly distributed state.”
Therefore, in this sulfidation step, in order to suppress a decrease in the nickel recovery rate, there is a demand for coping with the “nonuniformly distributed state” of the iron ions concentration in the sulfidation starting solution.
Patent Document 1 discloses a method for recovering sulfide precipitates in which the molar ratio S/(Ni+Co) is controlled to a value of 1.05 or less equivalent to that of the sulfide generated by using hydrogen sulfide, desirably to a value near 1 which is a stoichiometric composition of NiS and CoS, as a method for precipitating and recovering a nickel and/or cobalt sulfide by adding alkali sulfide to an acidic aqueous solution containing nickel and/or cobalt. Specifically, a method is disclosed in which after the inside of a reaction container is set to be under a non-oxidizing gas atmosphere, alkali sulfide is added to an aqueous solution and the sulfide is formed by precipitation, while maintaining the oxidation-reduction potential (Ag/AgCl electrode standard) at −300 mV to 100 mV.
However, in the method of Patent Document 1, new alkali sulfide such as Na2S or NaHS is used, resulting in a significant increase in cost. From this point, there is a demand for a method for manufacturing a nickel and cobalt mixed sulfide by which a decrease in nickel recovery rate can be suppressed even when the iron ions concentration in the sulfidation starting solution is increased without using new alkali sulfide.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2006-144102